Insulating materials are used in many industries, including the iron and steel, aluminum, glass, cement, refining and petrochemical, and waste energy industries. Insulations, particularly high temperature insulations, are widely used in furnaces, particularly ceramic firing furnaces.
Insulation boards and other geometric forms can be generally divided into two categories, shaped insulation bricks and unshaped insulation bricks. Shaped insulation include alumina bricks, alumina-carbon bricks, magnesia-carbon bricks and coatings, and gunning castable insulation. Shaped insulation can be used for items such as slide gates, nozzles, porous plugs and shrouds for continuous casting. Low temperature class insulations, insulations used at temperatures below about 1000.degree. C., are generally shaped insulations. Unshaped insulations are not preshaped, but are machined into a usable form. High temperature class insulations, also called refractories, which are used at temperatures above about 1000.degree. C., are generally unshaped and machined into usable form.
High temperature insulations are mostly used in furnaces. As such insulations are not preshaped, it is desirable that the insulation material be lightweight and easily machined. Additionally, as furnaces often rapidly cycle between temperatures it is important that the refractory material be resistant to thermal shock. Further, insulating bricks will preferably have a low thermal conductivity, as this increases energy efficiency of the furnace.
Pure alumina or zirconia bricks may be used in furnaces operating at temperatures above about 1500.degree. C., however, such bricks are dense, difficult to machine, and not particularly resistant to thermal shock. Additionally, such bricks have high thermal conductivity, which diminishes the energy efficiency when used as a furnace lining.
Insulation materials for use in furnaces may be comprised with low thermal connectivity materials such as alumina, alumina silica or combinations thereof. These materials are porous, as this lowers the thermal conductivity. Such materials are generally low in density and easily machinable. However, when such insulations are prepared for use at temperatures above 1200.degree. C., they usually contain fibers of alumina, silica, alumina silicates or mixtures thereof.
Insulation bricks may be made with high temperature advanced composites which comprise materials such as silicon nitride, silicon oxynitrides, silicon aluminum oxynitrides, silicon carbide and titanium diboride. Carbides, borides and nitrides have been incorporated into composite materials in order to increase the high temperature mechanical properties of the insulation. However, such composite materials are often difficult to machine. Further, alumina, mullite, zirconia and other ceramics which are substantially free of fibers generally have poor resistance to thermal shock and will crack or shatter easily upon rapid cooling. The use of fibers in composite insulation materials have been shown to increase resistance to thermal shock, and therefore fibers are generally included in such compositions.
Fibrous insulation materials generate airborne fibers during manufacture, use and disposal. Unfortunately, airborne fibers pose potential health hazards for the fibers are irritants to the skin, eyes and upper respiratory system. Generally about one fiber/cc per 8-hour exposure is the level commonly thought to be dangerous. Therefore, the manufacture, use and disposal of fiber containing insulation materials require the use of protective gear such as face and body masks.
Additionally, many insulating materials contain petroleum-like materials called pitch or resin. Pitch is normally used as a binder. However, the use of pitch as a binding material has certain disadvantages. Pitch is easily oxidized, and once the binding material in the insulation is oxidized there is no bonding to prevent grains from slag attack. Pitch is environmentally hazardous because it contains volatile compounds. Further, the use of pitch makes it difficult to incorporate strength improvement agents into the insulating material, and the use of pitch in insulating materials containing carbides or borides increases the sintering temperature required during the synthesis of the insulation.
Therefore, it would be advantageous to formulate an insulating material which is substantially free of fibers and substantially free of pitch, and which is low in density, resistant to thermal shock, easily machinable and has a low thermal conductivity.